IGBTs are widely used in high-power switching equipment, such as high-voltage DC (HVDC) equipment, including voltage source converters. In such applications, an IGBT is typically combined with a diode arranged in parallel and conducting in the reverse transistor direction (“free wheel diode”).
RC-IGBTs are chips in which a transistor is combined with a diode conducting in the reverse transistor direction. Numerous chip designs have been proposed and used within this concept, which has also been further developed relatively recently into the BIGT (see M. Rahimo et al., “Realization of high output power capability with the bi-mode insulated gate transistor (BIGT)”, 13th European Conference on Power Electronics and Applications, EPE '09, (8-10 Sep. 2009)). As used herein, the term RC-IGBT covers both conventional RC-IGBTs and BIGTs. A particular advantage of BIGTs over other RC-IGBT designs is their soft turn-off behaviour in both transistor and diode mode. It also appears possible to reduce the reverse recovery losses in BIGTs further than in earlier RC-IGBT technology.
Aspects of the gate-emitter voltage control in RC-IGBTs are discussed in WO 2010/149431 and WO 2010/149432. These documents describe a gate-emitter voltage controller adapted for a mode-dependent response to an ON command. In practice, the controller responds differently depending on the value of a voltage derived from the collector-emitter voltage and indicating whether the device is conducting in the forward or reverse mode. A voltage divider, which may include one or more high-voltage diodes, a Zener diode and/or a voltage source in addition to high-voltage resistors, provides this indication voltage as a constant fraction of the collector-emitter voltage. It is only in the forward conduction mode that the controller turns the transistor on in response to said ON command. According to WO 2010/149431, the turn-on gate voltage is released if the controller detects that the current has changed its direction or if it receives an OFF command.
A further article by Rahimo et al., “A high current 3300 V module employing reverse conducting IGBTs setting a new benchmark in output power capability”, Proceedings of the 20th International Symposium on Power Semiconductor Devices & ICs (18-22 May 2008), describes a technique for controlling an RC-IGBT in its reverse conducting mode. During conduction in the diode mode, the gate-emitter voltage is kept negative to store charge in the device and achieve a low forward voltage drop. When the diode is about to be turned off through turn-on of the opposite IGBT, a short positive gate-emitter voltage pulse is applied to the conducting diode to minimize the stored charge. A timing of the pulse that minimizes the reverse recovery charge and current may be chosen.